Neuronal ceroid lipofuscinoses (NCLs), a category of hereditary cerebellar ataxia, cerebellar abiotrophy or cerebellar cortical degeneration, is a group of monogenic inherited neurodegenerative storage diseases, found in men and mice, that are characterized by psychomotor retardation, blindness and premature death. In this clinically and pathologically heterogenous group of diseases, progressive loss of neuronal populations occurs within the cerebellum, specifically the Purkinje neurons and the granular cell layer. Loss of neuronal populations from other areas of the central nervous system also may occur, depending on the particular disease. This neuronal degeneration results in the insidious development of ataxia and a number of other neurological signs (depending on the disease) that progress at varying rates until the patient is incapacitated.
Similar hereditary NCLs have been reported in several domestic animals species including cattle (Houweling et al. (2006) Biochim. Biophys. Acta 1762:890-897), goat (Fiske and Storts (1988) Vet. Pathol. 25:171-173), sheep (Tammen et al. (2006) Biochim. Biophys. Acta 1762:898-905; Frugier et al. (2008) Neurobiol. Dis. 29:306-315), cat (Weissembock and Rossel (1997) J. Comp. Pathol. 117:17-24; Bildfell et al. (1995) Vet. Pathol. 32:485-488; Nakayama et al. (1993) J. Vet. Med. Sci. 55:829-831; Green and Little (1974) Can. J. Comp. Med. 38:207-212) and certain dog breeds.
The first American Staffordshire Terriers (AST) displaying the clinical and histopathological features of an adult-onset cerebellar cortical degeneration or cerebellar ataxia were diagnosed in 2002 and 2003 (Thibaud et al. European Society of Veterinary Neurology 15th Annual Symposium, Philadelphia Pa., September 2002; Hazli{hacek over (c)}ek et al. (2003) Schweiz Arch. Tierheilkd 145:369-375). Affected dogs showed first clinical signs from 18 months to 9 years of age with the majority of dogs presented to veterinarians between 4 and 6 years of age. They showed first stumbling, truncal sway and ataxia exacerbated by lifting the head up and negotiating stairs, progressing to an obvious ataxia characterized by dysmetria, nystagmus, coarse intention tremor and falling with transient opisthotonus. Most dogs survived for 2 to 4 years before they were euthanized while they became totally unable to walk without falling repeatedly. Histophathologic findings included loss of Purkinje cells and thinning of the molecular and granular layers.
In 2004, Siso and collaborators showed that a cerebellar cortical degeneration seen in five ASTs and eight American Pit Bull Terriers (APBT) bred in the US could be classified as a neuronal ceroid lipofuscinosis (Siso et al. (2004) Acta Neuropathol. 108:386-392). Indeed, they performed light microscopic, immunocytochemical and ultrastructural investigations and found that the neuronal degeneration not only affected Purkinje cells of the cerebellum but that diffuse nerve cell loss and gliosis were observed in the thalamic nuclei. They reported an accumulation of yellow-brown granules within neurones. These granules exhibited yellow autofluorescence under ultraviolet light; they were positive following periodic-acid-Shiff (PAS) and Sudan black staining thus indicating that the neuronal degeneration in these five ASTs was associated with neuronal ceroid lipofuscinosis
The same year, Olby and collaborators determined that the more consistent mode of inheritance of the disease in the established pedigree of AST from the US was autosomal recessive (Olby et al. (2004) J. Vet. Intern. Med. 18:201-208) but the implicated gene was not identified.
Dogs displaying symptoms of the disease are therefore homozygous for the causal mutation, and healthy carrier dogs clinically undetectable and being heterozygous for the causal mutation also exist. No predicting examination is currently available to determine the status of a dog towards NCL. Indeed, only the confrontation of the breed and the age of the dog, the presence of characteristic symptoms and an MRI exam of the brain enable for diagnosing NCL in an AST.
The appearance of this disease and the apparent increase in its prevalence are of great concern for the American Staffordshire Terrier and the American Pit Bull Terrier breeds because it is an incapacitating disease for which no cure is known. Moreover, the late onset of signs results in affected dogs being bred before they develop ataxia, potentially causing wide dissemination of the disease within the breed.
Accordingly, there is a need for genetic methods to identify healthy carrier and affected dogs before breeding, in order to stop the spreading of the disease. Determination of the causal mutation of the disease is necessary to design such a method.
In humans, NCLs have been attributed to mutations in six genes, namely PPT1(palmitoyl-protein thioesterase 1), TPP1 (tripeptidyl peptidase 1), CLN3 (ceroid lipofuscinosis, neuronal 3), CLN5, CLN6 and CLN8 (Siintola et al. (2006) Biochim. Biophys. Acta 1762:857-864). Mutations in three additional genes are involved in NCLs in animals. In White Swedish Landrace sheep, a mutation in the cathepsin D (CTSD) gene causes autosomal recessive congenital ovine NCL (Tyynela et al. (2000) EMBO J. 19:2786-2792). Furthermore, null mutations in Clcn3 (chloride channel 3), Ppt2 (palrnitoyl-protein thioesterase 2) and Ctsf (cathepsin F) are responsible for various forms of NCLs in mice (Yoshikawa et al. (2002) Genes Cells. 7:597-605; Gupta et al. (2001) Proc. Natl. Acad. Sci. USA 98:1.3566-13571; Tang et al. (2006) Mol. Cell. Biol. 26:2309-2316). Canine NCLs have been diagnosed in at least 18 breeds, and previous studies have found a CLN8 missense mutation associated with NCL in English Setters (Katz et al.
(2005) Biochem. Biophys. Res. Commun. 327:541-547), a CLN5 nonsense mutation associated with NCL in Border Collies (Melville et al. (2005) Genomics 86:287-294), a CTSD missense mutation associated with NCL in American Bulldogs (Awano et al. (2006) Mol/ Genet. Metab. 89:254-260), and a TPP1 single nucleotide deletion at CLN2 associated with NCL in Dachshunds (Awano et al. (2006) Mol. Genet. Metab. 89:254260). No common mutation was therefore identified according to the breed.